This invention relates to the preparation of pre-coated ferrous-alloy components. More particularly, the present invention relates to the use of a surface preparation as a preliminary step in a pre-coating process to improve the corrosion protection and other properties of coated ferrous-alloy components.
Aircraft manufacturers use a variety of different ferrous and non-ferrous metals in the fabrication of aircraft components. Commonly assigned U.S. Pat. No. 5,614,037 discloses a method for pre-treating aluminum and aluminum-alloy articles to obviate the use of wet sealants and other coatings for protection against corrosion damage.
Ferrous-alloy metals such as carbon steels and aircraft-quality low-alloy stainless-steels, such as, for example, Aermet 100, HY-TUF™, 300M, H-11, HP9-4-30, 52100, 1095, 4130, 4135, 4140, 4330V, 4340, 6150, 8740, 18-8, 17-4PH, 17-7PH, 15-5PH, PH 13-8Mo, PH 15-7Mo, A-286, etc. are often used as primary structural aircraft components. Typically, these ferrous-alloy components, including fasteners, bearing, struts, etc., are often protected from wear and corrosion by applying an overplate of cadmium in time-consuming combination with other protective finishes such as chrome plate. These fasteners are often installed using a labor-intensive, time-consuming, and, consequently, very costly wet-sealant process.
However, cadmium is suspected of being a carcinogen and is a known toxic element. Cadmium and cyanide, used in the electrodeposition of cadmium, have been listed as two of the seventeen chemicals targeted by the U.S. Environmental Protection Agency (EPA) for reduction. Additionally, many cadmium-plating specifications require a chromate coating or other subsequent seal or finish to improve corrosion resistance. This adds another toxic metal (hexavalent chromium) that must be treated before discharge. Many European nations have passed legislation restricting import of products with cadmium, and the European Economic Community (EEC) has prohibited use of cadmium-plated products. Such concerns have resulted in the search for a replacement coating or finish.
Although several candidate coatings have been identified, no single replacement coating or system has been found that meets all of the engineering requirements. Earlier tests that formed the foundation for U.S. Pat. No. 5,614,037, as well as also U.S. Pat. Nos. 5,858,133, 5,922,472, 5,944,918, 6,403,230, and 6,221,177 demonstrated an equivalent level, if not improved, of corrosion protection was achieved by pre-coating non-ferrous-alloy components in lieu of the inferior practice of applying wet sealant to the component during its assembly. See also commonly assigned U.S. Pat. Nos. 6,274,200 and 6,494,972.
However, it has been shown that, with respect to pre-coated components having different ferrous-alloy substrate materials, while the surrounding structural components are adequately protected to equivalent levels from corrosion attack, the coating itself that is applied to some of the components in known pre-coating processes may be adversely affected from an appearance standpoint and may interact with the substrate material to a degree that is visually perceptible. Over a prolonged period of time, the possibility exists that the adverse effect of this interaction could not only manifest itself in the coating's visual appearance but may also have an effect on the coating's integrity, possibly leading to a compromised corrosion protection condition.